The present invention pertains to an apparatus and method for proportioning defactant and surfactant to be used with foam generating equipment to allow continuous, precision proportioning of defactant and surfactant in dust control, benefication, flotation, and flocculation processes. The instant invention also provides a method of controlling the production and flow of foam in process applications by the addition of defactants to deactivate residual surfactants contained in the foam dust control method as set forth in U.S. Pat. No. 4,400,220.
Froth producing compounds, such as ("MICROBOND.RTM.", surfactant of the DeTer Company), wetting agents, or foam producing surfactants are used in foam generators to produce the film comprising the small bubble foam.
The term "surfactant", as used herein, refers to surface-active agents which comprise polar compounds consisting of an amphophilic molecule (a molecule with a hydrophilic head attached to a long hydrophobic tail). The hydrophilic group may be anionic, cationic, amphoteric or nonionic. As used in the present invention, the addition of a surfactant to a liquid system reduces the liquid's surface tension and in the presence of gas, promotes foaming. Typical foaming agents include silicone glycols, alkylbenzene sulfonates, alcohol ethoxylates, phosphate esters, betaines, alkylphenol ether sulfates, alkylaryl sulfonates, and other formulations including the foaming agent used in the preferred embodiment MICROBOND.RTM. (of the DeTer Company).
The term "defactant", as used herein, refers to surface active agents which causes an increase in the surface tension of the liquid causing the collapse of bubbles in foam. Defoamers commercially available include silicon compounds, alcohols such as methanol or ethanol, and various salts. Specific examples include polypropylene glycols, polyglycol esters, EO-PO copolymers, polydimethyl siloxane sulfonates, ethoxylated fatty alcohols, polyethylene glycol, polysiloxane blends, and dimethyl silicones. Commerically available defoaming agents are also sold under the following trade names: ("ANTIFOAM AF", of Dow Corning Corporation), ("AZ-10A, AZ-20L", A-Z Products, Inc.), ("CC. 101, 103", Custom Chemical Co.), ("ARIZONA 302, 305", Arizona Chemical Co.), ("PLURONIC L-61 SURFACTANT", of BASF Corp., Chemicals Div.), ("OA-5, OA-5U", Cities Service Co.), ("H-10, B ANTIFOAM EMULSION", Dow Corning Corp.), ("AF60, AF70, AF9000, GE10, GE60, GE66, GE70, GE71", General Electric Co.), ("KENNESAW 81", Kennesaw Chemical Corp.), ("FOAMASTER DEFOERS", Henkel Corp.), ("NEPCO 8050, 8171", Diamond-Shamrock Chemical Co.), ("PC-1244" Monsanto Chemical Co.), ("UNITOL ANTIFOAMS", Union Camp Corp.), ("UNITO DSR", Union Camp Corp. ), ("SAG SILICONE ANTIFOAMS", Union Carbide Corp. ), ("WESTVACO L-6", West Virginia Pulp end Paper Co.), and ("PLURONIC L-61, TETRONIC 1101", Wyandotte Chemical Corp.).
The term "foam", as used herein, designates a mixture of liquid, gas, and a surfactant that gives the liquid a film strength which permits the formation of long lasting bubbles when the mixture is agitated to convert it into a mass of bubbles. The liquid used is normally water, and the gas is usually air, because these ingredients are of low cost, but other gas and/or liquid can be used when compatible with the surfactant. The strength of the film depends upon the characteristics of the surfactant, and the amount of the surfactant in the liquid-gas mixture.
Small bubble foam generators are known, as described in U.S. Pat. Nos. 3,811,660 and 4,400,220 which are hereby incorporated by reference. In accordance with U.S. Pat. No. 3,811,660, it is necessary to cause the air, water, and surfactant (surface active material) mixture to be subject to "substantial agitation" to produce small bubble foam. This process is performed by causing the mixture to flow at or above a minimum velocity through a pipe, hose or foamer, a unit having "tortuous passages", or through a foamer as shown in my U.S. Pat. No. 4,207,202.
These generators produce foam with small bubble size at fairly high rates, which is useful for many applications, such as the use of foam for dust suppression. However, many applications for "small bubble" foam require very small flow rates of foam. The apparatus and method for controlling the flow of foam at low flow rates are described in my U.S. Pat. Nos. 4,830,737 and 5,019,244, which are hereby incorporated by reference.
To control dust, it is necessary for the small particle to contact a bubble of the foam and burst the bubble. As the bubble bursts, the gas in the bubble escapes, explodes, and the liquid film of which the bubble was made coats the particle. Particles as small as one micron are readily wetted using the small bubble foam.
The foam produced by the method described in U.S. Pat. Nos. 3,811,660 and 4,400,220 contain small bubble foam. The small bubble foam produced by equipment constructed according to U.S. Pat. Nos. 3,811,660 and 4,400,220 have bubbles from about 50 to 200 micron diameter (0.05 to 0.20 mm) (0.002 to 0.008 inches) when first ejected from the foam generator wherein some of the bubbles coalesce forming bubbles of about (0.015 inches) in diameter. These bubbles exist in a matrix consisting of water and surfactant in the form of highly stressed films surrounding small pockets of air.
The foam bubbles are destroyed by contact with the particles. Generally, finely divided solids suspended in a medium tend to agglomerate wherein the particles touch and form a rather loose and open structure. However, where the dust particles are mixed in with larger particle of various sizes, the wetted particles must then be brought together, made to contact larger particles, or brought into contact with a wetted surface.
For instance, if the foam is injected into a free-falling aggregate (at a transfer point between belts, for example, or injected into a crusher along with the aggregate), the mechanical motion of the aggregate will provide the required particle-to-particle contact. When the foam is injected into an aggregate which is all fines (one to two hundred micron), some means must be provided to cause the wetted particles to coalesce. This is readily accomplished by the use of a cyclone, as disclosed in U.S. Pat. No. 4,000,992.
Minerals such as coal are often subject to numerous flotation and benefication processes to remove impurities and control dust during processing (mining, grinding, transporting, and cleaning). After the initial application of a foaming agent or surfactant to the mineral ore, even though the material may lose its original moisture content, a small amount of surfactant is adsorbed on the mineral particles and remains with the treated material, Additional applications of foaming agents in subsequent processing operations are often ineffective due to incompatibility "chemistry" between the foaming agents. In the case of a flotation or frothing process, if the initial surfactant is similar to chemicals used in the subsequent treatment process, the quantity of surfactant affects the ratio of normally applied chemicals which may reduce the effectiveness of the chemicals and increase the cost of the treatment.
Residual surfactant contained in the small bubble foam also dries forming a film on the mineral ore. Even though the material may lose its original moisture content, the surfactant material is still present. Upon rewetting of the treated material, such as in a flotation process, the surfactant becomes reactivated reducing the surface tension of the water. Agitation of the surfactant containing water tends to form froth and small bubbles. This property has a detrimental effect on the structural strength of a product such as Portland Cement because entrainment of the air bubbles weaken the structural integrity of the cement.